The seminar starts every Thursday at 17:00 CEST (Central European Summer Time) and is live-streamed on our youtube channel.

Once the live stream has finished, we will make all video recordings and .pdf versions of the presentation slides available here.

Thu 2020-07-02 17:00 CEST

Quantum Science Seminar #11: Nanophotonics
Photo: Humboldt-Foundation/Elbmotion
Arno Rauschenbeutel
Humboldt University
Berlin — Germany
Revisiting Light-Matter Interaction in Quantum Nanophotonics
The interaction of a single-mode light field with a single atom or an ensemble of atoms can be described by a simple Hamiltonian and has been extensively studied. Nonetheless, the vector properties of light in conjunctions with the multilevel structure of real atoms and their collective response result in rich and surprising physics. In our group, we investigate this subject matter using nanophotonic components, such as subwavelength-diameter optical fibers and whispering-gallery-mode resonators, for interfacing light and atoms. I will present three effects that we observed in experiments with these systems and that go beyond the standard description of light-matter coupling. First, transversally confined light can locally carry transverse spin angular momentum, which leads to propagation direction-dependent emission and absorption of light. Second, when imaging an elliptically polarized emitter with a perfectly focused, aberration-free imaging system, its apparent position differs from the actual position. Third, an ensemble of atoms can change the photons statistics of light transmitted through the ensemble. There, depending on the number of coupled atoms, a collectively enhanced nonlinearity leads to pronounced photon bunching or anti-bunching.

Thu 2020-06-25 17:00 CEST

Quantum Science Seminar #10: Ultrafast Science
Anne L’Huillier
Lund University
Lund — Sweden
Atomic photoionization using attosecond pulses
Since the beginning of the millennium, physicists know how to generate pulses of light of attosecond duration [1], thus gaining access to this incredibly short time scale. In this presentation, we will show how attosecond pulses bring new light on ultrafast electron dynamics in atomic photoionization. We use attosecond pulse trains together with a weak infrared probe to measure both amplitude and phase of photoionization matrix elements. Our method, which combines high temporal and spectral resolution [2], allows us to gain new insights on photoionization dynamics, including electron correlation and spin flip induced by spin-orbit interaction. In another experiment, we characterize an electron wavepacket near an autoionizing resonance in helium using a Wigner representation [3], and retrieve the corresponding time-dependent density matrix.

Thu 2020-06-18 17:00 CEST

Quantum Science Seminar #09: Molecules
Jun Ye
JILA, NIST and University of Colorado
Boulder — Colorado — U.S.A.
A Fermi gas of polar molecules from 3D to 2D
Quantum degenerate gases of polar molecules provide a new platform for quantum science [1]. A Fermi gas of KRb molecules is fully thermalized with atom-molecule interactions and characterized using thermometry based on suppressed density fluctuations [2]. To demonstrate the full potential of strong dipolar interactions in the molecular gas, we apply external electric fields to explore the exciting interplay between molecular interaction dynamics and dissipation. By confining KRb to two dimensional optical traps with a perpendicular electric field [3], we demonstrate greatly enhanced elastic collisions with strong suppression of inelastic loss, with their ratio reaching 100. The favorable 2D dipolar interactions have led to rapid thermalization and evaporation of molecules.

Thu 2020-06-11 17:00 CEST

Quantum Science Seminar #08: Quantum Transport
Thierry Giamarchi
University of Geneva
Geneva — Switzerland
Quantum transport, low dimensions and cold atomic systems
Measuring the transport properties of a system connected to reservoirs is one of the most common and most useful probe of the properties of a solid. Besides its practical interest transport in quantum systems poses fundamental and challenging theoretical questions, since it is one of the simplest realizations of an out of equilibrium phenomenon. I will review these issues, in particular in the case of one- and quasi-one (e.g. ladders) systems. In such systems we know that interactions lead to unusual ground states [1-2] and remarkable properties such as spin-charge decoupling, which of course has strong consequences for transport properties, in particular decoupling charge and spin transport [3]. I will connect these theoretical questions with experiments done in the context of cold atomic systems that provide novel ways to probe such physics [4].

Thu 2020-05-28 17:00 CEST

Quantum Science Seminar #07: Photonics
Cristiane de Morais Smith
University of Utrecht
Utrecht — Netherlands
Quantum Fractals
The human fascination for fractals dates back to the time of Christ, when structures known nowadays as a Sierpinski gasket were used in decorative art in churches. Nonetheless, it was only in the last century that mathematicians faced the difficult task of classifying these structures. In the 80’s and 90’s, the foundational work of Mandelbrot triggered enormous activity in the field. The focus was on understanding how a particle diffuses in a fractal structure. However, those were classical fractals. This century, the task is to understand quantum fractals. Last year, we experimentally realized a Sierpinski gasket using a scanning tunneling microscope to pattern adsorbates on top of Cu(111) and showed that the wavefunction describing electrons in a Sierpinski gasket fractal has the Hausdorff dimension d = 1.58 [1,2,3]. However, STM techniques can only describe equilibrium properties.
Now, we went a step beyond and using state-of-the-art photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].

Thu 2020-05-21 17:00 CEST

Quantum Science Seminar #06: Molecules
Kang-Kuen Ni
Harvard University
Cambridge — Massachusetts — U.S.A.
Combining Chemistry and Physics in Ultracold Polar Molecules
Advances in quantum manipulation of molecules bring unique opportunities, including the use of molecules to search for new physics, harnessing molecular resources for quantum engineering, and exploring chemical reactions in the ultra-low temperature regime. In this talk, I focus on the latter topic where we work toward a detailed microscopic picture of molecules transforming from one species to another and reveal several surprises along the way. By preparing quantum-state-selected KRb molecules at a temperature of 500 nK, we observed reactions proceeding through a long-lived intermediate, which provides a handle to steer with light the reaction pathway away from its natural course. Despite the long lifetime that might allow thermalization, our measurements indicate that ergodicity does not hold for all degrees of freedom.

Thu 2020-05-14 17:00 CEST

Quantum Science Seminar #05: Quantum Optics
Klaus Mølmer
Aarhus University
Aarhus — Denmark
Quantum interactions with radiation that moves
How does a quantum system interact with a travelling pulse of quantum radiation, prepared, e.g., in a number state or a coherent state of light? You may think that this problem has been text book material for decades along with detailed solutions for the case of simple, few level systems. But, in fact, it has not. While crucial for multiple effects in quantum optics and for the entire concept of flying and stationary qubits, quantum optics textbooks do not provide a formal description applicable to this foundational and elementary interaction process. After the introduction of a new (and simple) theoretical formalism that, accounts for the interaction of travelling pulses of quantized radiation with a local quantum system, I shall discuss applications of the theory to quantum pulses of optical, microwave and acoustic excitations and show examples of relevance to recent experiments with qubits and non-linear resonators.

Thu 2020-05-07 17:00 CEST

Quantum Science Seminar #04: Polaritons
Photo: Olivier Ezratty pour
Jacqueline Bloch
Center for Nanoscience and Nanotechnology
C2N — Université Paris Saclay — CNRS
Palaiseau — France
Quantum fluids of light in semiconductor lattices
When confining photons in semiconductor lattices, it is possible to strongly modify their physical properties and explore the physics of a variety of Hamiltonians. Photons can behave as finite or even infinite mass particles, photons can propagate along topological edge states without back scattering, photons can become superfluid and behave as massive interacting particles. These are just a few examples of exotic properties that we can imprint into quantum fluids of light in semiconductor lattices. Such manipulation of light presents not only potential for applications in photonics, but great promise for fundamental studies of driven dissipative systems. After a detailed introduction to quantum fluids of light, I will illustrate the variety of physical systems we can emulate with this photonic platform by presenting some recent experiments related to quasi-crystals, helical photons, and photonic graphene. Perspectives in terms of quantum correlations will be discussed.

Thu 2020-04-30 17:00 CEST

Quantum Science Seminar #03: Quantum Optics
Darrick Chang
Institute of Photonic Sciences (ICFO)
Barcelona — Spain
The maximum refractive index of an atomic medium
It is interesting to observe that all optical materials with a positive refractive index have a value of index that is of order unity. Surprisingly, though, a deep understanding of the mechanisms behind this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single, isolated atom is known to have a giant optical response, with a resonant scattering cross section that far exceeds its physical size.

Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and near-field interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strong-disorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and near-field interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one near-resonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for real-life solids.

Thu 2020-04-23 17:00 CEST

Quantum Science Seminar #02: Quantum Computing
Roee Ozeri
Weizmann Institute of Science
Rehovot — Israel
Trapped-ion quantum computing: a coherent control problem
Several systems have been investigated in the last couple of decades as possible platforms for the realization of a quantum computer. Among the different systems examined, trapped-ion systems have thus far demonstrated the highest fidelity quantum gates and very long coherence times. However, scaling trapped-ion quantum computers to large numbers of qubits has proven to be a difficult problem. In this talk I will review the quantum tool-box of trapped-ion quantum computing and discuss the coherent control techniques that were developed in recent years that render trapped-ion quantum gates robust against errors and allow for quantum computing on long chains of ions.

Thu 2020-04-16 17:00 CEST

Quantum Science Seminar #01: Quantum Simulation
J. Ignacio Cirac
Max-Planck-Institute of Quantum Optics (MPQ)
Garching b. München — Germany
Analog Quantum Simulation: from physics to chemistry
Many-body systems are very hard to simulate due to the explosion of parameters with the system size. Quantum computers can help in this task, although one may need scalable systems, something that is out of reach in the short run. An attractive alternative is provided by analog quantum simulators which, even though they are not universal, they can still be tuned to study interesting problems. Atoms in optical lattices seem to be ideally suited for that task. Most of the proposals of such simulators have focused so far on condensed matter or high energy physics problems. In this talk I will show how one can extend the range of problems to other scenarios, especially to quantum chemistry.